A three-phase chain-link converter consists of three phase legs where each phase consists of a number of series-connected cell converters. The phase legs of series-connected cell converters can be connected either in delta- or in wye-configuration. Each cell of the converter includes a DC capacitor where the cell energy is stored. The DC voltage of the DC capacitor shall be sufficient to generate an AC voltage component at the cell AC terminal. The energy of the DC capacitor can be extracted from the feeding network, which is the case for voltage source converter (VSC) applications, or from external sources. The external sources may be batteries to be charged and discharged. The power injected into the DC side of the cell shall ideally be equal to the power output on the AC side of the cell.
The power of the sources connected to each cell may not be equal, in terms of sign and magnitude. It is normally assumed for a chain-link converter that the power output of each individual cell shall be equal to each other except when DC voltage unbalances caused by disturbances must be cancelled by control actions. To achieve this equal output, e.g. a battery connected to a cell may be used to adjust for non-equal power of the external power sources.
Also, the external power sources may not be located at the same points which may influence the cost aspect of the system if the power source is connected to the converter cells by means of long cables.
WO 2010/102667 relates to a modular voltage source converter (VSC) comprising one or more phases. Each of the phases comprises converter cell modules connected in series. At least one converter cell module in a phase is assigned a separate distributed energy source in a separate housing. At least one converter cell module in a phase is capable of generating active power, and the converter cell module with an assigned energy source may be placed together at distant places, separate from the other converter cell modules in a phase. This is said to be an advantage compared to having the energy supply for active power generation at a common place, as then there will e.g. be a need for long cables to connect converter cell modules to energy sources.